Friday, December 20, 2013

Awards Roll in for IceCube

The IceCube observation of astrophysical neutrinos has been nicely (and quickly!) recognized. 



First, Physics World magazine recognized us their 2013 "Breakthrough of the year".   IceCube wont he top honors, beating out nine other stories, including the Rex-Isolde observation that some atomic nuclei are pear-shaped.

Then, Scientific American selected IceCube as one of their top 10 science stories of 2013. 

All-in-all, it was a pretty darn good month for IceCube. 


Wednesday, November 27, 2013

Big Bird joins Bert and Ernie


Bert and Ernie have company!  IceCube has found another PeV neutrino, event more energetic than Bert or Ernie.   The pictures below shows the IceCube standard event display, with side and top views.  Unfortunately, some of the optical modules see so much light that you can’t see what’s happening near the neutrino interaction.  In fact, the photodetectors in the optical modules are badly saturated.  The two event displays below show the side and top views of Big Bird; the enormous hit in the nearby DOMs overwhelms the image.  Each colored sphere represents an optical module that observed light; the colors show the relative time of the hit, from red (earliest) through orange and yellow.  The size of the spheres shows the amount of light detected.

Like Bert and Ernie, it is well contained within the detector, with no sign of early hits that might signal an entering muon; it is quite clearly a neutrino induced cascade (particle shower).  


The event was found by LBNLs Lisa Gerhardt, on her last day working on IceCube, before she moved to a position focused on high-performance computing - still at LBNL, but now at the National Energy Research Supercomputer Center).   Talk about going out in style!

I was privileged to be able to first show this event at the 2013 International Cosmic Ray Conference (ICRC), in Rio de Janeiro, Jul2 2-9th.  I would have loved to blog about it earlier, but the only available reference is the writeup of my talk.  Because that writeup covers the evidence for extra-terrestrial neutrinos (my last two posts), we agreed not to post it publicly until the Science paper appeared.  Now that the Science paper is out, the writeup is available on the Cornell preprint server as arXiv:1311.6519.  
 

Unfortunately, the collaboration decided not to release any information on the event energy or direction, pending a systematic analysis, but it is a nice little monster.   As you should expect, we are working hard on these analyses, so I should be able to say more in the not too distant future.


Friday, November 22, 2013

Public Access to "Evidence for High-Energy Extraterrestrial Neutrinos at the IceCube Detector"

Our Science paper has now been posted to the Cornell preprint server (arXiv),  and is available at

http://arxiv.org/abs/1311.5238

Click on the 'PDF' link on the upper right to get the complete text.

For those of you not familiar with science publishing, the preprint server contains the full text (as of today) of  892,854 scientific papers, covering most of physics and math, as uploaded by the authors.  In high-energy and nuclear physics, virtually every relevant paper is uploaded to the server, usually at the same time that it is submitted to a journal or conference proceedings.  Although it does not provide the benefits of peer review, it does provide quick access to the latest work - papers appear between 24 and 48 hours after they are uploaded.

The server also has a search facility, but, for particle/nuclear physics, I prefer INSPIRE .  INSPIRE isn't perfect, but it has broader coverage than the arXiv, since it provides overage before 1995, when the arXiv was established.  The arXiv coverage was also very spotty for the first ~ decade of operation.  INSPIRE also covers most of the relevant articles that are not uploaded to the arXiv.





Thursday, November 21, 2013

"Evidence for high-energy extra-terrestrial neutrinos" on the cover of Science



This weeks issue of Science has a the IceCube paper that we’ve all been waiting for:  Evidence for high-energy extra-terrestrial neutrinos.  The paper describes a follow-on analysis to Bert and Ernie (our two 1-PeV neutrinos.   The analysis was designed to find more events like Bert and Ernie.  It did not.  It did, however, find 28 events that appeared to come from interactions within the detector, with no evidence of an incoming muon track as expected from downward-going cosmic ray muons.    One of the events even made the cover of Science.   Unfortunately, Science requires a subscription, but we will release freely-available version of the paper this afternoon; I'll post the URL when it comes out.

The thing that makes this analysis so successful is that it brought together multiple techniques to reject most background and estimate the reset, leading to a convincing detection of a 4-sigma excess of events above the background level expected from atmospheric neutrinos. 

The first technique has been around since the first IceCube cascade analysis: using the edges of the detector for a veto, with a smaller fiducial (active) volume in the center.  This eliminates most background from downward-going muons entering the detector.  These downward-going muons outnumber the neutrinos by 500,000 to 1, and estimating the fraction that sneaks through the veto region is tricky, requiring voluminous simulations.   The new analysis uses a data-driven estimate instead.  The estimate uses two independent nested veto regions surrounding a smaller fiducial volume.  It counts events tagged in the outer veto which miss the inner veto to determine the veto miss fraction.

The other background is atmospheric neutrinos.   These are, on average, less energetic than the extra-terrestrial events.    The new analysis considers the expected energy spectrum, but it adds a new handle.  Energetic downward-going atmospheric neutrinos should be accompanied by a cosmic-ray muons which may trigger the veto mentioned above, so they are less likely to pass the final event selection.   The new study is the first one to search for downward-going cascades.   This atmospheric neutrino ‘self-veto’ probability is included in the background estimates.   The background estimates also took advantage of the latest IceCube measurements of the atmospheric neutrino rates. 

In total, our best estimate of the background was 12.1 events (including 1.5 ‘prompt’ atmospheric neutrinos from the decay of charmed particles), giving a significance as an extra-terrestrial signal ‘at the 4-sigma level.’   Of course, there are some caveats, but this looks like a fairly robust detection, especially with Bert and Ernie.

The energy spectrum of the events is shown in the figure above (the points with errors).  The blue histogram shows the atmospheric neutrino background, while the magenta and green lines include two estimates of the prompt atmospheric neutrinos; the shading shows the uncertainty.  The red shows the remaining downward-going muon background, while the grey line includes these backgrounds, plus an assumed astrophysical component.   The extra-terrestrial signal is significant starting at energies above 60 TeV.  The absence of events at energies much above 1 PeV is significant, indicating that the spectrum is cut off at very high energies (between 2 and 10 PeV); this may be a clue about the accelerators that produced the neutrinos.

Unfortunately, we don’t know where these neutrinos come from.   There is no statistically significant clustering in the sky map.


The apparent flux of extra-terrestrial neutrinos is toward the high end of current theoretical estimates, near the Waxman-Bahcall (WB) bound.  The WB bound is a calculation based on the measured cosmic-ray flux, assuming that, when these particles are accelerated, they interact with background gas or photons (light) in the accelerator, producing particles (pions) which decay, producing the neutrinos that IceCube observes.   Further studies, with more data, should give us clues which will help us located these accelerators.

For comparison, the only other observations of extra-terrestrial neutrinos have been from our Sun (created by the nuclear fusion that powers it) and a short burst of neutrinos when supernova 1987a exploded.  These neutrinos were all a million times lower in energy than the ones that IceCube observed.

Many  institutions have issued press releases  and feature stories about the paper,   A few of them are


My apologies for the length and technical level of this post, but this analysis is quite intricate, and I wanted to do it justice.

Monday, October 14, 2013

Shutting down Science

As most of you know, the U.S. federal government has largely shut down, except for services essential to preserve life or property.   Federal contractors, such as the Department of Energy laboratories are, so far, mostly remaining opening, burning through any carry-over funds.

Even though many institutions have remained open, the consequences for science (and, by analogy, for many federal activities) are severe.   Many scientific experiments are rather large, and it requires significant planning to do anything.  These efforts cannot 'turn on a dime.'   

This is particularly true for the U.S. Antarctic program.  Antarctic science is cyclical - almost everything happens during the brief Austral summer.  It is also incredibly dependent on an amazing logistics program, to make sure that everyone and everything needed makes it to Antarctica.  There is no corner store if you forget a pair of pliers. 

In a normal October, Lockheed-Martin Antarctic Support, the U.S. Air Force, and other logistics providers would be shipping supplies and people to Antarctica, opening up buildings, preparing to set up field camps, etc.  This is a prelude to the brief season when most of the science is done.  Now, mid-stride, they have to abandon a year of planning, stop moving people and material South, and prepare to reverse the process, while they determine what services are considered 'essential.'  Even if Congress and the President reach agreement, and re-open the federal government tomorrow, major damage will have been done to the season, and many (possibly even most) of the planned scientific studies will have to be scrapped or postponed. 

As a scientist, I am distressed by these abrupt cancellations.  It is disheartening to have our faces rubbed in the fact that our bosses (here, I mean Congress) really do not care what we do.  In this fight, science is so insignificant that it merits neither support nor opposition.

As a taxpayer, I am appalled by the enormous financial waste.  The NSF spends tens of millions of dollars to hire and train people, equip them and fly them to Antarctica.  Most of this money has already been spend, and many of the people are already in Antarctica.  Cancelling the season now would makes absolutely no sense, and would be a huge financial waste.

And, as a husband and father, I am concerned about my own future.  I am 'fortunate' to work in a division at a national laboratory that is still operating, so I am still being paid.  However,  if the shutdown continues, none of us know how long this will last.

Before I close, here are a two other blog posts on the shutdown:

From an anonymous woman who blogs from McMurdo Station: http://icewishes.wordpress.com/  
From the users group for Brookhavens Relativistic Heavy Ion Collider:  http://rhicusers.blogspot.com/





Thursday, April 25, 2013

Bert and Ernie Step Out

Last August, I posted a piece about  two high energy (PeV, 10^{15} eV neutrinos that IceCube had observed.  The neutrinos had been announced at Neutrino 2012, the main scientific conference for neutrino enthusiasts.  Now, we (IceCube) has written a paper about the two events, which has been posted to the Cornell preprint server at http://arxiv.org/pdf/1304.5356v1.  The events have not changed much, but we have spent the intervening months refining the reconstructions and evaluating the backgrounds.    We now know the energies much more accurately:  1.04 and 1.14 PeV respectively, with a 15% uncertainty.    1 PeV = 1,000,000,000,000,000 electron volts, compared to 120 electron volts for an electron moving through the wires in your house.

We are also hard at work at follow-up studies, but we're not ready to present these yet.

The two events have been given catchy names, which have been featured in a number of news & blog reports about the events.    The two event displays show the two events.  Each dot is an IceCube optical sensor that observed Cherenkov light from the charged particles produced in the neutrino interaction.  The colors show when the light arrived at the sensor (red = earliest, then yellow, green, blue...), and the size of the circle indicates the number of photons that were observed.






Monday, April 8, 2013

Dark Matter - after 80 years, a mystery endures

Dark matter is one of the enduring mysteries of the universe.  The first signals were observed by Fritz Zwicky back in the 1930's.  Zwicky and others observed that there was not enough visible mass in galaxies to provide the gravitational attraction required to keep them bound as they are.  By 'visible mass,' he meant the mass that could be seen i.e. stars.   He also determined that most of the'missing mass' was more broadly distributed than the visible mass.    Initially, this was thought to be likely due to dust clouds or other non-luminous mass.  However, over the past 80 years, scenarios where the hidden mass consists of 'normal' matter - protons, neutrons and electrons - have been largely ruled out. Among other things, if dark matter were 'normal,' it would have altered the density of heavy (heavier than hydrogen) nuclei produced in the early universe.   We have also observed many different signatures of dark matter, at very different distance scales - individual galaxies, galaxy clusters, and signs from the big bang.

For example, the recent findings from the Planck satellite (right) shows that dark matter is needed to explain the observed fluctuations in the cosmic microwave background radiation.  Further, this dark matter must be 'cold,' that is moving at much less than the speed of light.  In other words, it is relatively heavy; the most popular theories predict masses from 10 to 1,000 times the mass of a proton.   So, by process of elimination, most scientists believe that dark matter must be made of some form of as-yet-unseen particles.  A modification of the rules of gravitation cannot be ruled out, but we have not yet found a satisfactory modification.

There are many efforts to look for dark matter, using diverse techniques.  High-energy physicists are working hard, looking for signs that these particles have been produced in proton-proton collisions at the LHC.

Others physicists are looking for the signature of dark matter interacting in a laboratory detector - "direct detection."  In essence, a dark matter particle will bounce off a target nucleus, causing it to recoil.  The details of the recoil energy spectrum depend on the type of dark matter particle and its mass.  Because the cross-sections are tiny and because very little energy is transferred in these interactions, this is best done using large detectors cooled to cryogenic temperatures, and operated in deep underground laboratories, to avoid the background from cosmic rays. Many different types of dark matter detectors are being pursued, with rather different experimental strategies, involving different types of target material, and different strategies to observe the recoil energy.

Finally, many astrophysical experiments are searching for new signs of dark matter in the cosmos.  These searches rely on the idea that dark matter is likely to be its own antiparticle, so two dark matter particles can collide and annihilate, producing a shower of normal-matter particles, which can then be detected using conventional detectors.    The idea that dark matter is its own antiparticle may seem very strange, but it happens naturally in many theories, such as supersymmetry.   These searches look for dark mater annihilation anywhere that dark matter is expected to cluster.  In other words, anywhere that gravity is strong.  Locally, IceCube has searched for dark matter annihilation in the center of the Sun; a similar search from the center of the Earth is coming soon.   IceCube has also looked for signs of dark matter annihilation in the center and halo of our galaxy, and, coming soon, from nearby spheroidal dwarf galaxies.  The latter are of interest because they are believed to have a very high ratio of dark matter to normal matter.  Of course, for galactic searches, many other particles can be studied.   The Fermi observatory (right) has looked for photons coming from the galactic center and galactic halo.  Last year, there were some reports of photons with an energy of 130 GeV coming from the galaxy, but further instrumental studies are required to know if this is real.

The AMS experiment on  the space station (right)  has recently published a report of an excess of positrons (compared with theoretical expectations), with energies up to 350 GeV.  This excess is of particular interest because the positron fraction appears to increase with energy, while theory predicts that it should decrease.  Of course, the same behavior could be because there is a relatively local source of cosmic-ray positrons.


This is a lot of different approaches to dark matter.   One may wonder if there is a coherent strategy here.  The answer is 'sort-of.'   In the absence of a single clear idea what dark matter is, no single approach is known to work.  So, we are pursuing a large number of different approaches, based on what different scientists (and funding agencies) find attractive.  For the current scope of experiments, the multiple approaches are technically and financially feasible.  If none of the current efforts bear fruit, a larger experiment may be needed; this will require a clear strategy choice.  However, to inject a note of caution, different models of dark matter predict widely varying behaviors (interaction and annihilation cross-sections, etc.), and not all of these models lead to experimentally observable consequences, with current or planned future detectors. 

In short, dark matter is an enduring mystery.  After 80 years of effort in diverse areas, we know a great deal about its affect on the universe.  However, we still don't have any direct evidence for its existence, and we really don't know what it is.